In the field of conventional financial service equipments, a banknote having a foreign substance on a surface is detected generally by a thickness identification method, in which a mechanical device is contacted with the banknote. The identification method is implemented by a driving component and a driven component rotating with respect to each other. Specifically, the driving component is a driving shaft for detecting the thickness, and the driven component contacts a surface of a banknote so as to identify a thickness trace of the banknote. When contacting a foreign substance attached on the surface of the banknote, the driven component may have an abrupt height variation partially, and the thickness trace of the banknote may be slightly higher partially, identifying the banknote having a foreign substance attached on the surface.
However, due to the high requirement on manufacturing precision, and the extremely stringent requirement on machining precision of full jumping of the thickness driving shaft and the driven shaft, a mechanical contact-type thickness identification device has a high manufacturing cost. Further, a gap may exist between multiple driven components due to their large manufacturing volumes, causing a missing detection of the banknote partially, and reducing the identification rate for the banknote having a foreign substance attached.